Optical semiconductor devices (semiconductor light-emitting devices) provided with light emitting diode (LED) chips as light-emitting elements have been widely used in various display apparatuses. Such LED chips have light-emitting layers composed of p-n junctions between semiconductor layers deposited on crystal substrates.
Examples of the optical semiconductor devices include visible light emitting devices and high-temperature operating electronic devices composed of gallium nitride compounds such as GaN, GaAlN, InGaN, and InAlGaN. Furthermore, blue light emitting diodes and UV light emitting diodes are now being developed.
An optical semiconductor device is provided with an LED chip as a light-emitting element mounted on a light-emitting surface of a lead frame. The LED chip and the lead frame are electrically connected by wire bonding, and these are encapsulated by a resin that protects the light-emitting element and functions as a lens.
In recent years, white LEDs have drawn attention as new light sources, and the market will be greatly expanded mainly in illumination applications. White LEDs in practical use are categorized into GaN bare chips coated with YAG phosphors that emit white light by color mixing of blue light emitted by GaN and yellow light from the phosphors, and a package of red, green and blue three chips. Nowadays, a combination of an UV LED light source and a plurality of phosphors is also being developed in order to improve hue. In addition, endurance is required for LEDs for illumination applications.
Epoxy resins, which are transparent and exhibit good processability, are generally used as sealing materials for encapsulating light-emitting elements such as LED chips. In general, epoxy resins for LED encapsulation are composed of bisphenol A glycidyl ethers, methylhexahydrophthalic anhydride, and amine- or phosphorus-base hardeners. Unfortunately, these components generate carbonyl groups, which cause yellowing of the resins, by UV ray absorption. In order to solve this problem, use of hydrogenated bisphenol A glycidyl ether is proposed (Nonpatent Document 1). This resin, however, does not yet exhibit satisfactory properties.
Silicone resins are widely used to suppress yellowing or a decrease in brightness caused by UV rays. Silicone resins, which are highly transparent in the UV region, barely undergo yellowing and a decrease in transparency by UV rays. Disadvantages of silicone resins are low light output efficiency due to a low refractive index and poor adhesion with lead frames or reflectors due to low polarity.
In surface mount LEDs, reflow soldering is generally carried out. In reflow furnaces, workpieces are exposed to heat at 260° C. for about 10 seconds. Conventional epoxy resins and silicone resins undergo thermal deformation or cracking in some cases.
[Nonpatent Document 1] The New Energy and Industrial Technology Development Organization (NEDO) “Report on High-Efficiency Electro-Optic Conversion Compound Semiconductor Development, Plan for Light of 21C, 2001”